From 7502be3b0e09479b7cd88bc8d751143b8794f40e Mon Sep 17 00:00:00 2001
From: Luke Street <luke@street.dev>
Date: Thu, 23 Oct 2025 11:13:39 -0600
Subject: [PATCH 1/2] Add thread pool async I/O backend

---
 CMakeLists.txt              |   1 +
 src/async_io.cpp            |  39 +++++--
 src/async_io.h              |   1 +
 src/async_io_threadpool.cpp | 214 ++++++++++++++++++++++++++++++++++++
 4 files changed, 248 insertions(+), 7 deletions(-)
 create mode 100644 src/async_io_threadpool.cpp

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7e94d63..e180138 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -164,6 +164,7 @@ add_executable(wibo
     dll/version.cpp
     src/access.cpp
     src/async_io.cpp
+    src/async_io_threadpool.cpp
     src/context.cpp
     src/errors.cpp
     src/files.cpp
diff --git a/src/async_io.cpp b/src/async_io.cpp
index 1de8ea5..02b4e3b 100644
--- a/src/async_io.cpp
+++ b/src/async_io.cpp
@@ -38,18 +38,43 @@ class NoOpBackend : public wibo::AsyncIOBackend {
 
 namespace wibo {
 
-AsyncIOBackend &asyncIO() {
-	if (!g_backend) {
+using BackendFactory = auto (*)() -> std::unique_ptr<AsyncIOBackend>;
+
+struct BackendEntry {
+	const char *name;
+	BackendFactory factory;
+};
+
+static constexpr BackendEntry kBackends[] = {
 #if WIBO_ENABLE_LIBURING
-		g_backend = detail::createIoUringBackend();
-#else
-		g_backend = std::make_unique<NoOpBackend>();
+	{"io_uring", detail::createIoUringBackend},
 #endif
+	{"thread pool", detail::createThreadPoolBackend},
+};
+
+AsyncIOBackend &asyncIO() {
+	if (!g_backend) {
+		for (const auto &entry : kBackends) {
+			DEBUG_LOG("AsyncIO: initializing %s backend\n", entry.name);
+			auto backend = entry.factory();
+			if (backend && backend->init()) {
+				g_backend = std::move(backend);
+				break;
+			} else {
+				DEBUG_LOG("AsyncIO: %s backend unavailable\n", entry.name);
+				if (backend) {
+					backend->shutdown();
+				}
+			}
+		}
 	}
-	if (!g_backend->init()) {
-		DEBUG_LOG("AsyncIOBackend initialization failed; using no-op backend\n");
+
+	if (!g_backend) {
+		DEBUG_LOG("AsyncIO: no backend available; using no-op backend\n");
 		g_backend = std::make_unique<NoOpBackend>();
+		g_backend->init();
 	}
+
 	return *g_backend;
 }
 
diff --git a/src/async_io.h b/src/async_io.h
index 4674b37..4c52fd1 100644
--- a/src/async_io.h
+++ b/src/async_io.h
@@ -24,6 +24,7 @@ namespace detail {
 #if WIBO_ENABLE_LIBURING
 std::unique_ptr<AsyncIOBackend> createIoUringBackend();
 #endif
+std::unique_ptr<AsyncIOBackend> createThreadPoolBackend();
 
 } // namespace detail
 
diff --git a/src/async_io_threadpool.cpp b/src/async_io_threadpool.cpp
new file mode 100644
index 0000000..43782aa
--- /dev/null
+++ b/src/async_io_threadpool.cpp
@@ -0,0 +1,214 @@
+#include "async_io.h"
+
+#include "errors.h"
+#include "files.h"
+#include "kernel32/overlapped_util.h"
+
+#include <algorithm>
+#include <atomic>
+#include <condition_variable>
+#include <deque>
+#include <mutex>
+#include <optional>
+#include <thread>
+#include <vector>
+
+namespace {
+
+struct AsyncRequest {
+	enum class Kind { Read, Write };
+
+	Kind kind;
+	Pin<kernel32::FileObject> file;
+	OVERLAPPED *overlapped = nullptr;
+	void *buffer = nullptr;
+	DWORD length = 0;
+	std::optional<off_t> offset;
+	bool isPipe = false;
+	bool updateFilePointer = false;
+
+	explicit AsyncRequest(Kind k) : kind(k) {}
+};
+
+class ThreadPoolBackend : public wibo::AsyncIOBackend {
+  public:
+	~ThreadPoolBackend() override { shutdown(); }
+
+	bool init() override;
+	void shutdown() override;
+	[[nodiscard]] bool running() const noexcept override { return mActive.load(std::memory_order_acquire); }
+
+	bool queueRead(Pin<kernel32::FileObject> file, OVERLAPPED *ov, void *buffer, DWORD length,
+				   const std::optional<off_t> &offset, bool isPipe) override;
+	bool queueWrite(Pin<kernel32::FileObject> file, OVERLAPPED *ov, const void *buffer, DWORD length,
+					const std::optional<off_t> &offset, bool isPipe) override;
+
+  private:
+	bool enqueueRequest(std::unique_ptr<AsyncRequest> req);
+	void workerLoop();
+	static void processRequest(const AsyncRequest &req);
+
+	std::atomic<bool> mActive{false};
+	std::mutex mQueueMutex;
+	std::condition_variable mQueueCv;
+	std::deque<std::unique_ptr<AsyncRequest>> mQueue;
+	std::vector<std::thread> mWorkers;
+	std::atomic<uint32_t> mPending{0};
+	bool mStopping = false; // guarded by mQueueMutex
+};
+
+bool ThreadPoolBackend::init() {
+	bool expected = false;
+	if (!mActive.compare_exchange_strong(expected, true, std::memory_order_acq_rel)) {
+		return true;
+	}
+
+	unsigned int threadCount = std::thread::hardware_concurrency();
+	if (threadCount == 0) {
+		threadCount = 1;
+	}
+	threadCount = std::min(threadCount, 4u); // cap to avoid oversubscription
+
+	{
+		std::lock_guard lk(mQueueMutex);
+		mStopping = false;
+	}
+	mWorkers.reserve(threadCount);
+	for (unsigned int i = 0; i < threadCount; ++i) {
+		mWorkers.emplace_back(&ThreadPoolBackend::workerLoop, this);
+	}
+	DEBUG_LOG("thread-pool async backend initialized (%u worker%s)\n", threadCount, threadCount == 1 ? "" : "s");
+	return true;
+}
+
+void ThreadPoolBackend::shutdown() {
+	if (!mActive.exchange(false, std::memory_order_acq_rel)) {
+		return;
+	}
+
+	{
+		std::lock_guard lk(mQueueMutex);
+		mStopping = true;
+	}
+	mQueueCv.notify_all();
+
+	for (auto &worker : mWorkers) {
+		if (worker.joinable()) {
+			worker.join();
+		}
+	}
+	mWorkers.clear();
+
+	{
+		std::lock_guard lk(mQueueMutex);
+		mQueue.clear();
+		mStopping = false;
+	}
+	mPending.store(0, std::memory_order_release);
+	DEBUG_LOG("thread-pool async backend shut down\n");
+}
+
+bool ThreadPoolBackend::queueRead(Pin<kernel32::FileObject> file, OVERLAPPED *ov, void *buffer, DWORD length,
+								  const std::optional<off_t> &offset, bool isPipe) {
+	auto req = std::make_unique<AsyncRequest>(AsyncRequest::Kind::Read);
+	req->file = std::move(file);
+	req->overlapped = ov;
+	req->buffer = buffer;
+	req->length = length;
+	req->offset = offset;
+	req->isPipe = isPipe;
+	req->updateFilePointer = req->file ? !req->file->overlapped : true;
+	return enqueueRequest(std::move(req));
+}
+
+bool ThreadPoolBackend::queueWrite(Pin<kernel32::FileObject> file, OVERLAPPED *ov, const void *buffer, DWORD length,
+								   const std::optional<off_t> &offset, bool isPipe) {
+	auto req = std::make_unique<AsyncRequest>(AsyncRequest::Kind::Write);
+	req->file = std::move(file);
+	req->overlapped = ov;
+	req->buffer = const_cast<void *>(buffer);
+	req->length = length;
+	req->offset = offset;
+	req->isPipe = isPipe;
+	req->updateFilePointer = req->file ? !req->file->overlapped : true;
+	return enqueueRequest(std::move(req));
+}
+
+bool ThreadPoolBackend::enqueueRequest(std::unique_ptr<AsyncRequest> req) {
+	if (!running()) {
+		return false;
+	}
+	if (!req || !req->file) {
+		return false;
+	}
+
+	{
+		std::lock_guard lk(mQueueMutex);
+		if (mStopping) {
+			return false;
+		}
+		mQueue.emplace_back(std::move(req));
+		mPending.fetch_add(1, std::memory_order_acq_rel);
+	}
+	mQueueCv.notify_one();
+	return true;
+}
+
+void ThreadPoolBackend::workerLoop() {
+	while (true) {
+		std::unique_ptr<AsyncRequest> req;
+		{
+			std::unique_lock lk(mQueueMutex);
+			mQueueCv.wait(lk, [&] { return mStopping || !mQueue.empty(); });
+			if (mStopping && mQueue.empty()) {
+				break;
+			}
+			req = std::move(mQueue.front());
+			mQueue.pop_front();
+		}
+
+		if (req) {
+			processRequest(*req);
+		}
+		mPending.fetch_sub(1, std::memory_order_acq_rel);
+	}
+}
+
+void ThreadPoolBackend::processRequest(const AsyncRequest &req) {
+	if (!req.file || !req.file->valid()) {
+		kernel32::detail::signalOverlappedEvent(req.file.get(), req.overlapped, STATUS_INVALID_HANDLE, 0);
+		return;
+	}
+
+	files::IOResult io{};
+	if (req.kind == AsyncRequest::Kind::Read) {
+		io = files::read(req.file.get(), req.buffer, req.length, req.offset, req.updateFilePointer);
+	} else {
+		const void *ptr = req.buffer;
+		io = files::write(req.file.get(), ptr, req.length, req.offset, req.updateFilePointer);
+	}
+
+	NTSTATUS completionStatus = STATUS_SUCCESS;
+	size_t bytesTransferred = io.bytesTransferred;
+
+	if (io.unixError != 0) {
+		completionStatus = wibo::statusFromErrno(io.unixError);
+		if (completionStatus == STATUS_SUCCESS) {
+			completionStatus = STATUS_UNEXPECTED_IO_ERROR;
+		}
+	} else if (req.kind == AsyncRequest::Kind::Read && bytesTransferred == 0 && io.reachedEnd) {
+		completionStatus = req.isPipe ? STATUS_PIPE_BROKEN : STATUS_END_OF_FILE;
+	} else if (req.kind == AsyncRequest::Kind::Write && bytesTransferred == 0 && io.reachedEnd) {
+		completionStatus = STATUS_END_OF_FILE;
+	}
+
+	kernel32::detail::signalOverlappedEvent(req.file.get(), req.overlapped, completionStatus, bytesTransferred);
+}
+
+} // namespace
+
+namespace wibo::detail {
+
+std::unique_ptr<AsyncIOBackend> createThreadPoolBackend() { return std::make_unique<ThreadPoolBackend>(); }
+
+} // namespace wibo::detail

From 1a75143756f66fb817d68337ad71d4a91dbc59bb Mon Sep 17 00:00:00 2001
From: Luke Street <luke@street.dev>
Date: Thu, 23 Oct 2025 11:22:45 -0600
Subject: [PATCH 2/2] Make it work with older code

---
 src/async_io_threadpool.cpp | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/src/async_io_threadpool.cpp b/src/async_io_threadpool.cpp
index 43782aa..c427a7e 100644
--- a/src/async_io_threadpool.cpp
+++ b/src/async_io_threadpool.cpp
@@ -77,7 +77,7 @@ bool ThreadPoolBackend::init() {
 	for (unsigned int i = 0; i < threadCount; ++i) {
 		mWorkers.emplace_back(&ThreadPoolBackend::workerLoop, this);
 	}
-	DEBUG_LOG("thread-pool async backend initialized (%u worker%s)\n", threadCount, threadCount == 1 ? "" : "s");
+	DEBUG_LOG("thread pool backend initialized (workers=%u)\n", threadCount);
 	return true;
 }
 
@@ -176,7 +176,11 @@ void ThreadPoolBackend::workerLoop() {
 
 void ThreadPoolBackend::processRequest(const AsyncRequest &req) {
 	if (!req.file || !req.file->valid()) {
-		kernel32::detail::signalOverlappedEvent(req.file.get(), req.overlapped, STATUS_INVALID_HANDLE, 0);
+		if (req.overlapped) {
+			req.overlapped->Internal = STATUS_INVALID_HANDLE;
+			req.overlapped->InternalHigh = 0;
+			kernel32::detail::signalOverlappedEvent(req.overlapped);
+		}
 		return;
 	}
 
@@ -202,7 +206,11 @@ void ThreadPoolBackend::processRequest(const AsyncRequest &req) {
 		completionStatus = STATUS_END_OF_FILE;
 	}
 
-	kernel32::detail::signalOverlappedEvent(req.file.get(), req.overlapped, completionStatus, bytesTransferred);
+	if (req.overlapped) {
+		req.overlapped->Internal = completionStatus;
+		req.overlapped->InternalHigh = bytesTransferred;
+		kernel32::detail::signalOverlappedEvent(req.overlapped);
+	}
 }
 
 } // namespace